Syrian children on the outskirts of Madaya after being given permission to leave by government troops.
Photograph: Louai Beshara/AFP/Getty Images

The United Nations humanitarian affairs chief has called for the evacuation of about 400 people from Madaya hospital after aid trucks reached the Syrian town which has been cut off for months by fighting.

Inside Madaya: 'It feels deserted but you know there are people there'

This had to be done as soon as possible “or they are in grave peril of losing their lives”, O’Brien said, adding that efforts would be made to get ambulances to Madaya on Tuesday to evacuate the 400, if safe passage could be assured.

Forces loyal to the Syrian regime have enforced a tight siege of Madaya since July, and until now only one aid delivery had been allowed in, in October.

The UN security council discussed the situation in Syria at a meeting in New York on Monday called by New Zealand, Spain and France.

“The tactic of siege and starvation is one of the most appalling characteristics of the Syrian conflict,” New Zealand’s UN ambassador, Gerard van Bohemen, told reporters.

The US ambassador, Samantha Power, criticised the “grotesque starve-or-surrender tactics the Syrian regime is using right now against its own people”.

She told the UN general assembly: “Look at the haunting pictures of civilians, including children, even babies, in Madaya.

“These are just the pictures we see. There are hundreds of thousands of people being deliberately besieged, deliberately starved, right now. And these images, they remind us of the second world war.”

Syria’s UN ambassador, Bashar Ja’afari, denied anyone was starving in Madaya and blamed Arab television especially “for fabricating these allegations and lies”. Speaking at the UN headquarters, he blamed “armed terrorist groups” for stealing humanitarian aid and reselling it at prohibitive prices.

But O’Brien said all the evidence the UN had showed there had been severe malnourishment, severe food shortages and reports of people “who are either starving or indeed have starved and died”.

Aid agencies said it would take several days to distribute the aid in Madaya, near Damascus, and Fua and Kefraya, in northern Syria, and it would probably last a month.

The fates of the three towns have been tied for months, with aid deliveries only being allowed if they were simultaneous. Attempts by backers of the rebels and of the government to orchestrate a population swap have yet to succeed, but an evacuation of the wounded was agreed in late December.

Safiya Ghosn, a teacher who stood at the entrance of the town hoping to be evacuated, told the Associated Press: “I want out. There is nothing in Madaya, no water, no electricity, no fuel and no food.”

Speaking on Monday evening from inside Madaya, Pawel Krzysiek, a Red Cross spokesman, said: “The first impression is really heartbreaking. You see a lot of people on the street, some of them smiling and waving to us, but many just simply too weak with a very, very bleak expression, very tired.

“It’s really heartbreaking to see the situation of the people. A while ago I was just approached by a little girl and her first question was: ‘Did you bring food? I hope that you brought food to Fua and Kefraya and to Madaya because we are really hungry.’ And I believe her. She looked hungry.”

The delivery – orchestrated by the International Committee for the Red Cross, the Syrian Arab Red Crescent and the UN – took place at nightfall in Madaya. The first four trucks carried food, medical aid, blankets and other supplies for the town of more than 30,000 people. The ICRC said it would work overnight to offload the trucks, 40 of which had been sent to Madaya, and 21 to Fua and Kefraya.

“Our main message is that this is not the long-term solution, neither for Madaya nor Fua and Kefraya or any other besieged place,” Krzysiek told the Guardian in a phone interview. “The most important thing is that we are able to access these places on a regular basis.”

The ICRC said it would distribute medicine through local health authorities.

Krzysiek said residents faced myriad medical problems. “You have pretty much everything here – chronic diseases like diabetes, people weak, people swollen because they have to eat this water with spices.”

Residents in Madaya have said they have been forced to live on tree leaves, flavoured water and grass scoured from minefields, and children have been kept from succumbing to malnourishment with sugar serums. Several have died of hunger.

On Sunday five more people – a nine-year-old boy and four men – died of starvation, the international charity Médecins Sans Frontières said.

Aid agencies welcomed the arrival of aid in Madaya but said one-off deliveries would not save starving Syrians.

The UN says 4.5 million Syrians are living in besieged or hard-to-reach areas and desperately need humanitarian aid, with civilians prevented from leaving and aid workers blocked from bringing in food, medicine, fuel and other supplies. Activists say starvation sieges have been used as a deliberate strategy in the war, which has claimed more than a quarter of a million lives in nearly five years.